The price, after $1.6 million was spent last year, has increased by another $4.9 million, the palace confirmed Wednesday, bringing the total so far to $6.5 million (adding in the cost of asbestos removal takes the figure to $7.2 million).

Shouldering the bill is the U.K. taxpayer, via what is known as the Sovereign Grant, though the family, and in large part Prince Charles, were responsible for the fixtures and furnishings in the living section of the compound.

The apartment itself is, in fact, a four-story home, with up to 30 rooms, including nine bathrooms – furnishings for which Kate shopped for personally, it is believed.

Part of the renovation included creating what a palace source describes as a “family-style” kitchen for the royals’ day-to-day use, while the large original kitchen (which features a wall of ovens) will be kept for official functions when the couple entertains formally.

The bulk of the renovation budget has been spent on essential works, from costly asbestos removal (which cost $700,000 alone) to rewiring and plumbing. “There wasn’t even running water,” the senior palace source says.

In addition to shopping for furniture, the couple took items from the Royal Collection, choosing, for example, from 365 wooden wardrobes in storage.

The source stressed that the updating had not been “lavish.” “You would be surprised. It is a comfortable and orderly level of furnishing – you would feel at home there.” And the couple was “at pains to bear down on costs. The kitchen was their idea and they were perfectly happy to pay for it,” says another palace source.

The royal family is also privately paying for the improvements being carried out at the couple’s country home, Anmer Hall on Queen Elizabeth’s Sandringham estate. The 10-bedroom country mansion, complete with tennis court and swimming pool, was a gift from the Queen, 88.

Aides stress that Apartment 1a will be their official home for many years to come and had to be converted from offices into a suitable home. Moreover, it had not been renovated for at least 50 years, the last extensive update being in 1963.

A spokesman for the couple confirms, “This is the Duke and Duchess’s one and only official residence. It is here that they plan to stay for many, many years to come.” He added, “They were also at pains to ensure that the specification is not extravagant.”